The present invention relates to protective relay systems for use in protecting AC power distribution systems and more particularly to protective relay systems utilizing a communication channel for transmission of comparison signals between protective relays.
Relays are utilized in protective systems for AC power transmission lines to detect faults occurring within a designated portion, or protected zone, of an AC power transmission line. If such a fault is detected within the protected zone of a transmission line, the relay may generate a signal to operate circuit breakers in order to electrically isolate the protected zone from the rest of the power distribution system.
Zoned protection can be implemented by using a pilot relaying scheme. Such a scheme employs a communication channel in conjunction with protective relays in order to ascertain whether a fault is within the protected line or external to it. The protective relays are typically utilized in pairs, one at each end of the protected zone of the transmission line, which are linked by the communication channel. Such an arrangement is depicted in FIG. 1 which schematically shows a transmission line 10 having a first protective relay 12 at one end X of the protected zone X-Y ; and a second protective relay 14 at the other end Y of the protected zone X-Y of the transmission line 10.
The first relay 12 generates a comparison signal S.sub.x which is transmitted to the second protective relay 14 by means of a communication channel 16. Similiarly, the second protective relay 14 generates a comparison signal S.sub.y which is transmitted to the first protective relay 12 also by means of the communication channel 16. Each protective relay utilizes the received comparison signal in conjunction with a locally generated comparison signal in order to determine whether or not to generate a trip signal which will cause the operation of the circuit breakers.
Ideally, the comparison received signal should be identical to the signal transmitted by the remote relay in order to insure that the protective relaying scheme will operate reliably. One way to accomplish this is to provide an error free communication channel; however, in practice , the communication channel can introduce errors as a result of excessive noise due to, for example, faults in the transmission line, signal frequency translation, alien signals, spurious signals, etc. Errors which affect the received comparison signal can also be caused by saturation of the current transformers used in the relay system which causes non-fundamental frequency sinusoidal components in the secondary current. Errors can also be introduced by the occurrence of internal frequency transients in the protected zone which have a much higher frequency than the fundamental frequency of the system. Such errors could cause the protective relay to generate an erroneous trip signal or conversely to refrain from generating a trip signal upon occurrence of a fault within the protected zone. Therefore, it can be seen that errors attributable to the communication channel linking the protective relays can adversely affect the reliability and security of the protective relaying scheme.
Accordingly, it is an object of the present invention to increase the reliability of protective relaying systems employing a communication channel for transmission of signals between relays.
Another object of the present invention is to enhance the reliability of protective relay operation by minimizing the effect of errors in the communication channel linking the relays.
It is still another object of the present invention to enhance the reliability of protective relay operation in the presence of excessive noise in the communication channel linking the relays.
It is yet another object of the present invention to enhance the security of protective relaying systems when the secondary current exhibits considerable distortion or
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.